Although social network analysis has shaped many literatures across several disciplines, there remain many core elements of social networks that are incompletely understood. The critical determinants of social tie formation and the effects of social networks on health are two primary examples of current incomplete scientific understanding of social networks in need of further research attention. In particular, there is ver limited understanding of how genetic factors may interact with social structures, such as schools, occupations, neighborhoods, etc. to both influence social tie formation as well as impact health. In addition, it is unclear how existing social policies, such as the tracking of students within schools, may shape and constrain social network formation opportunities, and thus impact health. The overall aim of this proposal is to explore the interplay between genetic variation, social structure, social tie formation, and individual health outcomes. This proposal is innovative in that all our aims will provide some of the first available evidence of these question in the literature. Our interdisciplinary research team will be able to accomplish this by leveragin newly collected and unique data that contain several key ingredients only recently made available in the National Longitudinal Study of Adolescent Health (Add Health) and the Wisconsin Longitudinal Study (WLS): information on social network ties, health information, genetic information, and highly geographically clustered samples. The use of multiple complementary datasets also allows a core focus on replicating findings and extending empirical approaches consistent with the unique structure of each dataset. Using these data, we will address current gaps in the research knowledge base by pursuing the following aims: (1) Increase understanding of the level of genetic homophily within social networks and understand its sources and (2) Increase understanding of the interplay between school policies and genetic variation in determining health behaviors. Our first aim will both document the level of genetic homophily in adolescent friendship networks from two datasets collected nearly 40 years apart but will also test specific hypotheses regarding the potential sources of genetic homophily. Our first aim will focus on separating genetic homophily from behavioral homophily and separating the influences of preferences for social ties who are genetically similar with the influence of social structures (i.e. schools) that shape and constrain the opportunities to form social ties. Together, these aims will provide new evidence of the interplay between social networks, social structure and policies, genetics, and health. Our proposal has the potential to contribute to the theoretical literature on social networks as well as the empirical literature on consequences of social networks.